Dolores Beasley
Headquarters, Washington Aug. 9, 2001
(Phone: 202/358-1753)
Steve Roy
Marshall Space Flight Center, Huntsville, AL
(Phone: 256/544-6535)
Nancy Neal
Goddard Space Flight Center, Greenbelt, MD
(Phone: 301/286-0039)
Megan Watzke
Chandra X-ray Observatory Center, Cambridge, MA
(Phone: 617/496-7998)
RELEASE: 01-161
ASTRONOMERS GO BEHIND THE MILKY WAY TO SOLVE X-RAY MYSTERY
Through layers of gas and dust that stretch for more
than 30,000 light-years, astronomers using NASA's Chandra X-
ray Observatory have taken a long, hard look at the plane of
the Milky Way galaxy and found that its X-ray glow comes from
hot and diffuse gas. The findings, published in the August 10
issue of the journal Science, help to settle a long-standing
mystery about the source of the X-ray emission from the
galactic plane.
Scientists have debated whether the Milky Way plane's X-ray
emission was diffuse light or from individual stars. Armed
with Chandra, an international team led Dr. Ken Ebisawa of
NASA's Goddard Space Flight Center, Greenbelt, MD, zoomed in
on a tiny region of the galactic plane in the constellation
Scutum.
"The point sources we saw in the galactic plane were actually
active galaxies with bright cores millions of light-years
behind our galaxy," said Ebisawa. "The number of these
sources is consistent with the expected number of
extragalactic sources in the background sky. We saw few
additional point sources within our galaxy."
The observation marks the deepest X-ray look at the so-called
"zone of avoidance" -- a region of space behind which no
optical observation has ever been taken because thick dust
and gas in the spiral arms of the Milky Way galaxy block out
visible radiation. Infrared, radio and X-rays, however, can
penetrate this dust and gas. Detection of diffuse X-rays
emanating from the galactic plane, what we call the "Milky
Way" in visible light, indicates the presence of plasma gas
with temperatures of tens of millions of degrees Celsius.
Gas this hot would escape the gravitational confines of the
Milky Way galaxy under normal circumstances. The fact that it
still lingers within the galactic plane is the next mystery
to solve. One possibility, suggested by Ebisawa, is that hot
plasma may be confined to the Milky Way by magnetic fields.
The Chandra observation, conducted in February 2000, lasted
28 hours. The team observed what was known to be a "blank"
region of the galactic plane where the Japanese X-ray
satellite ASCA had previously observed but found no
individual X-ray sources.
The team also discovered 36 bright distant galaxies lurking
in the background of this section of the galactic plane,
while the foreground was devoid of stars or other individual
objects emitting X-rays. Chandra, and now the European XMM-
Newton satellite, are at long last beginning to collect light
from behind our galaxy. X-ray radiation from the 36 newly
discovered galaxies passes through the Milky Way on its
journey toward Earth. This light, therefore, carries the
imprint of all that it passes through and will allow
astronomers to measure the distribution and physical
condition of matter in our galaxy.
Participating in the Chandra observation and Science article
are Yoshitomo Maeda of Pennsylvania State University,
University Park; Hidehiro Kaneda of the Institute of Space
and Astronautical Science in Japan; and Shigeo Yamauchi of
Iwate University in Japan.
Chandra observed the galactic plane with its Advanced CCD
Imaging Spectrometer (ACIS) instrument, which was developed
for NASA by Pennsylvania State University, University Park,
and Massachusetts Institute of Technology, Cambridge. NASA's
Marshall Space Flight Center in Huntsville, AL, manages the
Chandra program, and TRW, Inc., Redondo Beach, CA, is the
prime contractor for the spacecraft. The Smithsonian's
Chandra X-ray Center controls science and flight operations
from Cambridge, MA.
Images associated with this release are available at:
http://chandra.harvard.edu
and
http://chandra.nasa.gov
-end-